1. Field of the Invention
The invention is related to the field of pressure measurement, and more particularly, to an improved differential pressure measurement.
2. Description of the Prior Art
Fluid flow meters are used to measure mass flow rates of flowing fluids, including gasses, liquids, or mixtures thereof. One application of a fluid flow meter is in measuring the mass flow rates of gasses used in semiconductor fabrication. The mass flow rate of such gasses is relatively low. In addition, the pressure may be relatively low. As a result, accurate and reliable measurement of such gas flows is problematic.
FIG. 1 shows a portion of a prior art fluid flow meter employing a differential pressure sensor. The prior art fluid flow meter can employ a venturi (shown) or other flow element that creates a pressure differential in the fluid flow. In the prior art, it has been common to use a differential pressure sensor to measure the pressure difference provided between a reduced pressure generated at the venturi throat and the normal flow pressure upstream or downstream of the venturi throat. The differential pressure sensor generates a differential pressure measurement that can be used to determine an associated mass flow rate of the flow fluid. If the flow rate is zero, then the differential pressure measurement will (or should) be zero.
The differential pressure sensor arrangement of the prior art has drawbacks. A differential pressure sensor typically works well for liquids and flow fluids having significant mass flows and significant flow pressures. This is because the difference in pressures will be significant and therefore easily measurable. However, for low mass flow rates and/or low pressure levels, a differential pressure sensor may not have sufficient sensitivity and accuracy. Differential pressure sensors with satisfactory small differential performance are very costly and therefore are not preferred for a low-flow gas flow meter application.
FIG. 2 shows a portion of a prior art fluid flow meter of an alternate approach. This prior art approach employs two absolute pressure sensors. Two absolute pressure measurements can be used to generate a differential pressure value by subtracting one absolute pressure measurement from the other absolute pressure measurement.
An absolute pressure sensor generates a measurement signal reflecting a quantification of an input pressure with reference to a vacuum. Absolute pressure sensors are available that provide a greater sensitivity and accuracy than a differential pressure sensor. The use of two absolute pressure sensors therefore may produce a more accurate result, as an absolute pressure measurement is easier to accurately obtain than a tiny pressure differential.
The two absolute pressure sensor arrangement of the prior art has drawbacks. The two absolute pressure sensors will need to be closely matched and will need to be calibrated to produce the exact same measurement for a particular pressure. The calibration of the two absolute pressure sensors must remain constant and accurate over the entire operational pressure measurement range. This may require expensive absolute pressure sensors that are manufactured to smaller tolerances. More frequent sensor calibration is consequently required.
In addition, variations in operational tolerances of the two pressure sensors may exist over the entire operational measurement range or only a portion, wherein calibration at a single pressure may not address drift or deviation over the entire operational range. Problematically, the resulting differential pressure value will be in error and in a flow meter application will generate an erroneous mass flow rate output.
What is needed, therefore, is an improved pressure differential measurement.